This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2009-142447 filed in Japan on Jun. 15, 2009, the entire contents of which are herein incorporated by reference.
The present invention relates to an image forming apparatus provided with a plurality of image carriers.
As an image forming apparatus, a so-called tandem image forming apparatus is conventionally known in which a plurality of images (for example, toner images) are formed by an image forming process of an electrophotographic method or the like, using a plurality of image carriers such as photosensitive bodies or the like that respectively correspond to the images, and the images are overlaid together. For example, when forming a full-color image, toner images of a plurality of mutually differing colors (ordinarily, color components of each of yellow (Y), magenta (M), cyan (C), and black (K)) are formed at a coordinated timing on the plurality of image carriers corresponding to the respective toner images, the respective toner images are transferred in a stacked manner to a transfer-receiving body such as an intermediate transfer body or a recording material, and when the transfer-receiving body is an intermediate transfer body, the toner images are furthermore transferred to a recording material.
In this conventional image forming apparatus, in some cases a first group to which at least one image carrier among the plurality of image carriers belongs, and a second group to which at least one image carrier among the remaining image carriers belongs, are driven independently.
Specifically, when monochrome image forming is performed, ordinarily a black image is formed individually without forming an image of another color. In this case, an image carrier corresponding to black and an image forming member (a member including a black development apparatus) for forming an image on the image carrier are driven with a different drive unit, such as a motor, than a plurality of image carriers respectively corresponding to other images (yellow, magenta, and cyan images) and image forming members (members including yellow, magenta, and cyan development apparatuses) for forming images on the image carriers.
On the other hand, although it is necessary to drive image carriers and image forming members for images in colors other than black (yellow, magenta, and cyan images), in order to reduce the number of drive components to achieve a smaller size for the image forming apparatus, it is possible to drive the respective image carriers for yellow, magenta, and cyan, which are driven simultaneously, and the image forming members corresponding to the image carriers, with a single drive unit. With such a configuration it is possible to reduce the number of components. A stepper motor is an example of a drive unit that drives a plurality of image carriers and image forming members.
Incidentally, even when a plurality of images are formed at a coordinated timing on the plurality of image carriers, image shift may occur when stacking the images of the respective image carriers. In order to prevent such image shift from occurring, it is important to precisely stack the images of the respective image carriers.
The occurrence of image shift is caused by, for example, rotational irregularity phase shift due to, for example, image carrier eccentricity, eccentricity of a drive transmission rotation member such as a drive gear that transmits rotational drive to an image carrier from a drive unit, and so forth.
For example, when a first group image carrier and a second group image carrier are driven independently, ordinarily, at the time of initial driving such as when power is turned on and at each instance of a predetermined period, the rotation phase of the first group image carrier and the second group image carrier are adjusted to a reference rotation phase, which is an optimal rotation phase where the rotational irregularity phase shift is as small as possible. However, even if phase matching is performed such that the rotation phase of the first group image carrier and the second group image carrier becomes the reference rotation phase, when an image is formed by driving only any one among the first group image carrier and the second group image carrier, in some instances the rotation phase of the first group image carrier and the second group image carrier may be completely different from the reference rotation phase. Alternatively, there may be instances when the rotation phase of the first group image carrier and the second group image carrier is shifted from the reference rotation phase, and thus image shift (phase shift) occurs.
In order to correct the rotation phase of the first group image carrier and the second group image carrier so as to become the reference rotation phase, conventionally, a detection sensor that performs phase matching of the rotation phase of the plurality of images that are stacked (that is, the plurality of image carriers) is provided for each image carrier, a rotation phase is detected by each of these detection sensors, a rotation phase difference of the detected rotation phase relative to the reference rotation phase is detected, and by thus changing at least one among the rotation timing of the first group image carrier and the rotation timing of the second group image carrier to correct the rotation phase of the first group image carrier and the second group image carrier, phase matching is performed. In this way it is possible to reduce the occurrence of rotational irregularity phase shift caused by eccentricity or the like.
Specifically, a detection sensor is provided for a first gear that transmits rotational drive to a first group image carrier (for example, a group carrier to which the black image carrier belongs), and a second gear that transmits rotational drive to a second group image carrier (for example, a group carrier to which the yellow, magenta, and cyan image carriers belong), the detection sensor detecting the rotational phase of the corresponding gear. Phase matching is determined by detecting the rotation phase of the first group image carrier and the second group image carrier with the respective detection sensors.
For example, JP 2006-84669A discloses a color image forming apparatus in which a photosensitive body is driven by a DC brushless motor having a Hall element via a drum gear provided with a rotation phase detection sensor, and rotation phase is detected in the drum gear and the motor.
However, at least one detection sensor that performs phase matching from the rotational irregularity of each image carrier is necessary for each image carrier that is driven. That is, for example, when a first group image carrier and a second group image carrier are independently driven, at least two sensors are necessary. Therefore, to that extent the apparatus configuration becomes more complex, and cost of the apparatus increases.